This invention relates to the conversion of optical radiation to electrical power or to an electrical signal.
A practical telecommunication system, such as one designed to carry telephone conversations between subscribers, includes several basic equipments: transducers (e.g., carbon microphones) for converting information encoded in acoustical energy (voice) to another energy form (e.g., electrical, optical) more suitable for transmission; transmission media (e.g., wire pairs, optical fibers) for carrying that energy from one location (e.g., subscriber premises) to another; switching equipment (e.g., crossbar, ESS) capable of interconnecting any pair of subscribers to one another over such transmission media; and supervisory signaling equipment (e.g., a bell or lamp and associated power sources) which, for example, alerts a subscriber to an incoming call.
As suggested parenthetically above, one type of telecommunication system relies, at least in part, on certain functions being carried out with energy in its optical form. Thus, there have been a number of speculative systems proposed for performing the switching function optically, but primary emphasis to date has been on fiber optic transmission systems in which electrical pulses representing encoded voice or data are used in a transmitter to pulse modulate a junction laser or LED acting as a carrier. The PCM optical signal is then carried over glass fibers to a receiver where it is detected by a Si photodiode (e.g., a p-i-n or avalanche photodiode of the type described in copending application Ser. No. 793,493 filed on May 4, 1977 by A. R. Hartman et al and assigned to the assignee hereof.) Fiber optic systems have been used to interconnect central offices but, as yet, have not been incorporated into the subscriber loop -- i.e., the interconnection between a central office and a subscriber's premises. For this reason, and at least one other, virtually no attention has been given to optically powering telephone equipment (e.g., a station set) -- it being assumed that local electrical power, such as that presently supplied by a central office battery for electrical systems, would be utilized.
Another reason why optical powering has been all but ignored arises from the low voltages available from photodiodes -- the front runner of photodetectors for optical telecommunication systems. For example, one of the more attractive photodiode candidates is a double heterostructure of AlGaAs-GaAs-AlGaAs layers in which radiation is made incident normal to the layers and generates photocarriers in the light absorbing GaAs layer. While this device is characterized by a high power conversion efficiency (e.g., 53% at 8150 Angstroms), it produces a maximum photovoltage of only about 85% of bandgap (i.e., yielding about 1.2 V) because of recombination in the GaAs layer. Yet, in a number of telephone equipments higher voltage is required (e.g., about 5 V for a bell ringer). The simple apparent solution would be to make a plurality of discrete photodiodes and connect them in a series, voltage adding relationship. Unfortunately, however, the size of such a configuration would require some form of beam splitting arrangement to direct the radiation to be detected (e.g., from an optical fiber) to each separate photodiode. To circumvent this size problem, the photodiodes might be stacked in an integrated structure, but the resulting p-n-p-n . . . configuration would result in a voltage drop across each interface between adjacent photodiodes. Such voltage drops subtract from the total photovoltage thus defeating the purpose of the series arrangement.